Memory allocation purity

[via Digitalmars-d]

On Monday, 19 May 2014 at 17:35:34 UTC, Steven Schveighoffer 
wrote:
> Returning the same immutable object, when called with the same 
> immutable parameters, should never cause a break in code, pure 
> or not.

This isn't at all obvious to me. Also I think the "coin flip 
trick" represent a class of algorithms that depend on imposing a 
sort order on objects. I can easily see an algorithm break if you 
sometimes receive the same object and sometimes receive a new 
object with the same values as parameters. That's how an 
optimizer works, sometimes it optimizes, sometimes not. If your 
algorithm depends on running something twice and then comparing 
the two results then it could easily break. In order to prevent 
this you would need to "box it as a value" and let the type 
system forbid object identity comparison.